The simplest solar cell of using monocrystalline silicon to convert solar radiation into usable energy is described below. The solar cell is based on monocrystalline silicon, usually a p-type semi-conductive monocrystalline silicon wafer, which is realized by doping boron compound into a monocrystalline silicon. Gaseous antimony doped in p-type thin film usually diffuse and form p-n junction on the silicon surface, changing the conducting species from holes into electrons, i.e. an n-type junction. The thickness of n-type deposition on silicon wafer is 0.5-3 μm. The thin film usually connects to a metal electrode (gold or its alloy).The back of the silicon wafer is completely covered with a metal electrode or silver deposited electrode.
Furthermore, the physics principle of solar cell is described below. When the device is activated by the radiation of the sun or artificial light, the photons absorbed by silicon result in unbalanced hole-electron pairs. At this moment, the electrons in the p-layer close to the p-n junction will drift to the boundary and be attracted into the n-type junction by the electric field; on the other hand, the holes carrier (p-type carriers) in the n-type junction on the silicon wafer surface will partially drift into the silicon wafer interior, i.e. the p-type junction. This drifts results in adding extra negative charges into the n-type junction and adding extra positive charges into the p-type junction. Thus the contact potential difference of p-type junction and n-type junction reduces, leading to a voltage in the outside circuit. The semiconductor power source described above has the n-type junction as the cathode and p-type junction as the anode. The effective working efficiency of the simplest framework of the aforementioned solar cell assembly is 15 to 16%.
The photoelectric effect occurring on silicon wafer under light can be described by volt-ampere characteristics equation:U=(KT/q)×ln[(Iph−I)/IS+IZ]where IS is the current supplied and Iph is the photoelectric current.The maximum power from the semiconductor per millimeter square can be written as Iph×U=X×IK3×UXX, where X is the proportional constant of volt-ampere characteristics, IK3 is the short-circuit current, and UXX is the floating voltage. The effective working efficiency of the simplest solar cell structure described above is 15-16%.
FIG. 1 shows that the basic framework of a conventional solar cell, wherein 1 is a p-type—monocrystalline silicon wafer, 2 is an n-type conductive layer, 3 is an electrode system, and 4 is an outer anti-reflection coating. The silicon wafer of the solar cell is usually covered with dustproof housing made of vinyl acetate or polycarbonate-like compound.
According to the solar radiation spectrum measured in the medium latitude region (at northern latitude 48°, for example), when the sun is 45° above the horizon, the maximum-energy wavelength of the solar spectrum reaching the earth surface is between 290-1060 nm. (It is worth noting that, when a solar cell works in the near-space environment, the complete spectrum also contains the short-wavelength radiation of UV and VUV and the medium-wavelength radiation of far-red longer than 1065 nm; on the other hand, when a solar cell works on the earth surface, the short-wavelength radiation will be absorbed by oxygen in the atmosphere, and the medium-wavelength radiation of UV will be largely absorbed by vapors).
It is also noting that the energy distribution of the solar radiation spectrum is uneven. The maximum energy of the solar radiation appears in the blue light (λ=470 nm). The solar radiation is reduced by 20% in the main section of visible light between the wavelength 500˜600 nm, and the corresponding radiation is half at λ=720 nm. Furthermore, the radiation at λ=1000 nm=1 μm is only ⅕ of the maximum value.
FIG. 2 shows the sensitivity of the standard spectral curve of a solar cell sample at each wavelength range corresponding to the solar spectrum. Compared the data of the solar radiation energy spectrum with the data in FIG. 2, it can be found that, at the wavelength range between λ=950˜980 nm, the solar cell assembly is most responsive with the maximum sensitivity because of the energy band structure of the monocrystalline silicon; the bandgap width of monocrystalline silicon is Eg=1.21 ev, corresponding to the wavelength of λ=950 nm. On the other hand, the solar cell assembly is virtually irresponsive to the ultraviolet (λ<400 nm), i.e. cannot absorb ultraviolet.
For a long time, researchers and producers have strived to overcome the defects and limitations described above. Chopr disclosed a solution in his article “Thin Film Solar Cells” (pages 378˜379, World Publish Ltd.), from which we developed a prototype. FIG. 3 shows a solar cell covered with a layer of monocrystalline ruby, which can enhance the absorption of the solar radiation in the range of 2.3ev˜3.2ev. The physical significance of this design is that, by coating a solar cell with a layer of monocrystalline ruby, the absorption of the solar radiation in the range of 2.3ev˜3.2ev will be enhanced, and the Cr+3 will be activated to induce d-d transitions and then cause the narrow band to emit light. Consequently, the peak wavelength of Cr+3 in the ruby corresponds to λ=695 nm, and thus the original solar radiation is moved to longer wavelength range, and the short-wavelength range of the radiation is completely shifted to the wavelength range of λ=700 nm.
In FIG. 3, the plot of photon energy verse absorptivity, the curve 2 is the absorptivity of the activated Cr+3, and curve 1 is the light emitting of an activated monocrystalline ruby. FIG. 3 is also marked with the carrier assembly coefficient (curve 3) of a monocrystalline silicon cell coated with an activatable ruby and the coefficient varies with whether the ruby layer is present. It is obvious that the carrier assembly coefficient of the directly-activated short wavelength of the solar radiation is 10˜20% higher than that of a light-emitting device operated by a ruby converter. The author of the article thus concluded that the efficiency of a monocrystalline silicon solar cell may still rise by 0.5˜2% with a ruby converter. Although some practical improvements have been made in the field of solar cell technology, some drawbacks including the high cost of monocrystalline ruby remain to be improved.